The synovial fluid found in mammalian joints functions as a lubricant and shock absorber. The most important component of the synovial fluid is sodium hyaluronate, which makes the greatest contribution to the mechanical properties of the fluid. Hyaluronic acid is a naturally occurring high molecular weight glycosaminoglycan having a repeating disaccharide unit of 2-amino-2-deoxy-3-o-(.beta.-D-glucopyranosyluronic acid)-D-glucose. The disaccharides are joined to form an unbranched, uncrosslinked polysaccharide chain by .beta.1.fwdarw.4 glucosidic bonds. In addition to its presence in synovial fluids, hyaluronic acid occurs in cell coats, pericellular gels, the extracellular matrix substance of connective tissues of vertebrates, the vitreous humor of the eye, human umbilical cord tissue, rooster combs and in some bacteria.
During inflammation of a joint caused, for example, by osteo- or rheumatoid arthritis both the molecular weight of hyaluronic acid and its concentration are reduced. This lowering of molecular weight decreases the ability of synovial fluid to act as a shock absorber, and thus the fluid does not provide adequate protection for the cartilage of the joint. Moreover, the lowering of molecular weight also reduces viscosity and thus promotes leakage from the joint. In the case of advanced arthritis the cartilage is eroded away, leading to pain when the joint is in motion (see for example, "The Merck Manual of Diagnosis and Therapy-16th Edition", p 1338-42).
One of the causes for the lowering of molecular weight and for the high rate of loss of sodium hyaluronate from the synovial cavity, is the degradation of the molecule by hydroxyl radicals. Hydroxyl radicals come from two sources. The primary source is white blood cells which enter the joints when they are inflamed, and release xanthine peroxidase and other enzymes to form superoxide anion, hydrogen peroxide and hypochlorite, which upon breakdown form hydroxyl radical. Another source for hydroxyl radical is the reduction of oxygen by reducing agents in the presence of iron. A common reducing agent in the body is ascorbic acid. Oxygen is reduced by iron(II) to form superoxide anion, which then reacts with iron (III) to form hydrogen peroxide. Hydrogen peroxide is reduced to hydroxyl radical.
The use of sodium hyaluronate of relatively moderate molecular weight as a supplemental synovial fluid in the leg joint of race horses has been reported (Balazs et al., J. Equine Vet. Sci., p. 217-228, 1985). However, synovial fluid in the joints of humans contains substantially higher molecular weight sodium hyaluronate than that of horses.
Solutions of sodium hyaluronate also have been tested as supplemental synovial fluid for human osteoarthritic joints by injection into the joints. Treatment of arthritis by injection of sodium hyaluronate has been disclosed by Weiss et al., Semin. Arthritis Rheum., 11, p. 143, (1981); Nakimi et al., J. Clin. Pharmcol. Therapy Toxicology, 20, p. 501, (1982); Grecomoro et al., Pharmatherapeutica, 5, p. 137, (1987) and Briganiti et al., Clinical Trials Journal, 24, p. 333, (1987). However, it has been reported that intra-articular injections of sodium hyaluronate solutions do not perform measurably differently from placebos (Dahlberg et al., in "Arthritis & Rheumatism" 37, p.521, 1994). Brown et al. in Ext. Physiol. 76, p.125, (1991), reported that the half-life of hyaluronic acid injected in a joint is only about 13 hours. Dahlberg, vide supra, has disclosed that a 13 hour half-life is short for therapeutic value. It is believed that the short half-life of injected hyaluronic acid is due in part to degradation by hydroxyl radicals (J. M. McCord, Science, 185, p.529, 1974).
This invention relates to novel ways of increasing the resistance of polysaccharides, in particular hyaluronic acid and sodium hyaluronate, to hydroxyl radicals by grafting them with antioxidants.